1. Field of the Invention
This invention relates to apparatus for writing on a recording medium of the type having a paper backed metallic layer which on contact with an energized stylus tip results in an electric erosion, or burn out, of the metalized layer at the point of contact so as to form an element of a printed character.
In the use of apparatus of this type, problems have been encountered in securing sharply defined character element by electro-erosion of the metalized layer because of wear by abrasion of the printing styli and also in securing a uniform contact pressure of the metalized paper with the styli. These problems have been known in the prior art, for example, from German Pat. No. 1,110,437 it is known that provision of a groove placed directly below the printing electrodes, or styli, serves to equalize contact pressure over the printing electrodes in a direction parallel to that of the movement of the recording medium. Another German Pat. No. 2,312,846 discloses printing styli having rounded surfaces at the point of contact with the recording medium for the same purpose.
U.S. Pat. No. 3,922,477 provides redundant electrodes, or styli, between adjacent writing electrodes where these redundant electrodes are at electrical ground potential to prevent bridging of adjacent writing electrodes by electrically conductive whiskers generated by the electro-erosion process.
2. Brief Summary of the Invention
Printing heads for electro-erosion printers generally have a plurality of writing electrodes or styli such that as the recording medium is pulled past the printing head and the writing electrodes are selectively impulsed with writing current, characters will be written element by element on the recording medium in a direction transverse or perpendicular to the direction of recording medium movement.
As the size of an electro-erosion printing head is increased so as to transversely contact a longer and longer printing line across the recording medium, contact pressure of the head against the medium departs from uniformity along the length of the printing line. Contact pressure varies from a high pressure at the ends of the line to a lower pressure in the middle. Abrasion of the writing electrodes is a function of contact pressure. Thus, the writing electrodes at the ends of the printing line are abraded more severely than those in the center. Also, should the printing head contain plural lines of styli, then those styli in the lines contacting the recording medium nearest the edges of the supporting groove beneath the recording medium are worn at a faster rate than the middle lines of writing styli.
Briefly, those problems are solved by the addition to the printing head of a number of blind or guiding electrodes placed at the ends of the long dimension of the printing head such that wear by abrasion affects these blind elements rather than affecting the writing styli themselves.
Accordingly, it is an object of this invention to reduce wear on the writing styli of an electro-erosion printing head.
Another object of the invention is to equalize and make the contact pressure of the recording medium more uniform along a line or lines of writing styli.
Another object of the invention is to improve the quality of electro-erosion printing over the lifetime of a printing head and to lengthen the useable lifetimes of electro-erosion printing heads.